1. Field of the Invention
The present invention relates to a flexible substrate, semiconductor device, imaging device, radiation imaging device and radiation imaging system. In particular, the present invention relates to a flexible substrate, radiation imaging device and radiation imaging system for use in a medical X-ray imaging system and an industrial non-destructive inspection system.
The radiation is defined in this specification to include X-ray, α-ray, β-ray and γ-ray.
2. Related Background Art
A radiation imaging device, particularly an X-ray imaging device for medical use has been required to be able to obtain animated images, to have high image quality, to be thin and to have a large input area. An inexpensive and thin-type X-ray imaging device having a large imaging area is also required in not only a medical imaging system but also in an industrial non-destructive inspection system.
An example of such an X-ray imaging device is one made to have a large imaging area by avoiding non-light receiving parts of a CCD (Charged Coupled Device) sensor from interfering with each other by providing steps on a fiber plate.
A schematic cross section of the conventional X-ray imaging device is shown in FIG. 14. FIG. 14 shows a fluorescent layer 3 comprising a scintillator for converting X-ray into a visible light, a fiber plate 2 comprising optical fibers for guiding the visible light converted with the fluorescent layer 3 to an imaging element 1 side, and the imaging element 1 (substrate) for converting the visible light transmitted to the fiber plate 2 into an electric signal.
The fiber plate 2 is formed with an inclination to the imaging element 1 in this X-ray imaging device, in order to provide a control device (not shown), which controls reading of electric signals from each imaging element 1 by addressing each imaging element 1 between two adjoining imaging elements 1. Since no X-ray impinges on the control device, noises by the incident X-ray can be suppressed from generating by using the fiber plate 2 as described above.
FIG. 15 shows a schematic perspective view of another conventional X-ray imaging device. The same reference numerals are given in FIG. 15 to the same parts as those in FIG. 14. In the X-ray imaging device shown in FIG. 15, three imaging elements 1 are arranged as a set, for example, by changing the length of the fiber plate 2, and a step is provided for each set to provide a control device for each imaging element 1. However, since the X-ray may impinge on the control device provided at the periphery depending on the size of the fluorescent layer, a X-ray shielding member made of, for example, lead should be provided around the imaging elements at the periphery for preventing the X-ray from impinging thereon.
However, the cost increases for the X-ray device shown in FIG. 14, since machining for cutting the fiber plate 2 aslant is difficult in addition to a small number of yield per one lot. Furthermore, light transmission efficiency becomes poor in each fiber in the fiber plate 2 and sensitivity of the sensor decreases by providing a slope.
While two fiber plates 2 with 2×2 blocks are bonded in FIG. 14, an area of 100×100 mm2 is a limit obtainable by using the currently available fiber plate 2. However, when 3×3 blocks of fiber plate are used by changing the slope of the fiber, the fiber plate located at the periphery has a light transmittance inferior to that of the fiber plate located at the center of the pixels in each imaging element, thereby resulting in uneven output signals from each imaging element.
In the X-ray imaging device shown in FIG. 15, on the other hand, the X-ray imaging device becomes large and heavy by providing the X-ray shielding member made of lead. In addition, since strict accuracy is required for positioning between each step and imaging element, the number of manufacturing steps increases while requiring a highly precise positioning instrument.
FIG. 16 is a schematic cross section of a conventional X-ray imaging device having a good workability in the manufacturing process that is suitable for solving the problems of large size, heavy weight and high cost without decreasing sensitivity of the X-ray imaging device.
The X-ray imaging device shown in FIG. 16 comprises a fluorescent layer (wavelength conversion device) 3 as a scintillator for converting the X-ray into a detectable wavelength light such as a visible light, fiber plates 2 comprising a plurality of optical fibers for guiding the light converted with the fluorescent layer 3 to an imaging element 1 side as well as shielding members for shielding the X-ray that remains not converted with the fluorescent layer 3, an adhesive 7 for bonding adjoining fiber plates 2 with each other, a transparent adhesive 6 for bonding the fiber plate 2 with the imaging elements 1, imaging elements 1 for converting a light into an electric signal, flexible substrates 4 for exporting the electric signal from the imaging elements 1 to the outside, a bump 5 for electrically connecting the flexible substrates to the imaging elements 1, a printed circuit board 12 as a read device to which the flexible substrate 4 is connected, a protective sheet 8 made of aluminum for protecting the fluorescent layer 3, a base substrate 10 for mounting the imaging element 1, a base case 11 for holding the base substrates 10, a case cover 9 provided on the base case 11, a spacer 13 provided between the imaging element 1 and fiber plate 2 for ensuring a give space, and a seal resin 15 for isolating the imaging element 1 from the external environment.
The problems arising in the X-ray imaging device in FIGS. 14 and 15 are solved in the X-ray imaging device having the construction described above by providing the control circuit between the pixels in each imaging element 1.
Also, the flexible substrate 4 is bent and inserted through adjoining plural imaging elements 1 for electrically connecting the printed circuit board 12 to the imaging element 1.
The X-ray imaging device shown in FIG. 16 is manufactured by bonding a set of the fiber plates, in which a plurality of fiber plates 2 are bonded with the adhesive 7, with a plurality of imaging elements 1 with the transparent adhesive 6.
Otherwise, the device may be manufactured by bonding a plurality of units of the X-ray imaging device by taking the size of the imaging element 1 or fiber plate 2 as a reference.
FIG. 18 is a schematic cross section in which the area Y in FIG. 16 is enlarged. In FIG. 18, the reference numeral 401 denotes an inner lead, the reference numeral 402 denotes a base film as a film, the reference numeral 403 denotes a cover film, and the reference numeral 105 denotes an organic insulation layer for preventing a short circuit between the end portion of the imaging element 1 and the inner lead 401 and for protecting the end portion of the imaging element 1 from being broken. The flexible substrate 4 comprises the inner lead 401, base film 402 and cover film 403.
The conventional method for connecting the bump 5 and flexible substrate 4 shown in FIG. 18 will be described using FIGS. 19A and 19B.
FIGS. 19A and 19B are schematic cross sections showing the step for connecting the bump 5 and flexible substrate 4 shown in FIG. 18.
At first, the organic insulation layer 105 is formed with a thickness of 25 μm. Then, the bump 5 is formed on the imaging element 1 by a stud bump method or plating for electrically connecting the bump 5 with the flexible substrate.
Then, the bump 5 is joined with the inner lead 401 by, for example, intermetallic bonding using an ultrasonic wave. The total thickness of the flexible substrate is adjusted to be about 50 μm.
Subsequently, a jig 19 is allowed to move toward holding tables 17 and 18, or the holding tables 17 and 18 are allowed to move toward the jig 19 while holding the imaging element 1 with the holding tables 17 and 18.
However, the base film 402 cannot be extended onto the substrate in the conventional flexible substrate 4, since the base film 402 constituting the flexible substrate 4 is formed with a thickness larger than the thickness of the bump 5 as an external connection terminal on the imaging element 1 (substrate). Accordingly, an organic insulation layer 105 should be independently provided in order to prevent the inner lead 401 from forming a short circuit when it is bent.
Positioning was also necessary for suitable positional relation between the base film 402 and bump 5 before bending the flexible substrate 4.
FIG. 20 shows a schematic cross section of the flexible substrate formed by favorably positioning the relation between the base film 402 and bump 5.
FIGS. 21A and 21B show schematic cross sections of the flexible substrate 4 when the positional relation between the base film 402 and bump 5 is not appropriate.
As shown in FIG. 21A, a short circuit is formed by making the inner lead 401 have contact with the substrate 1, or the inner lead may be broken at the edge of the substrate 1, when the inner lead 401 contacts the bump 5 at a position remote from the substrate 1 with a larger distance than a prescribed position.
As shown in FIG. 21B, when the inner lead 401 contacts the bump 5 at a position closer to the substrate 1 than the prescribed position, on the other hand, the base film 402 is pushed up causing a tensile force in the inner lead 401, and creating a possibility of breaking the inner lead 401.
In FIG. 16, a high quality and high resolution sensor may be provided with little scattering of light when the gap between the fiber plate 2 and imaging element is as narrow as possible, since the light emitted from the fiber plate 2 is a diffused light. Although it is preferable for the inner lead 401 to be made thin, a thin inner lead has the tendency to break when positioning is not proper.